1 Dynamic DMA mapping using the generic device
2 ============================================
3 4 James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
5 6This document describes the DMA API. For a more gentle introduction
7of the API (and actual examples), see Documentation/DMA-API-HOWTO.txt.
8 9This API is split into two pieces. Part I describes the basic API.
10Part II describes extensions for supporting non-consistent memory
11machines. Unless you know that your driver absolutely has to support
12non-consistent platforms (this is usually only legacy platforms) you
13should only use the API described in part I.
14 15Part I - dma_ API
16-------------------------------------
17 18To get the dma_ API, you must #include <linux/dma-mapping.h>. This
19provides dma_addr_t and the interfaces described below.
20 21A dma_addr_t can hold any valid DMA address for the platform. It can be
22given to a device to use as a DMA source or target. A CPU cannot reference
23a dma_addr_t directly because there may be translation between its physical
24address space and the DMA address space.
25 26Part Ia - Using large DMA-coherent buffers
27------------------------------------------
28 29void *
30dma_alloc_coherent(struct device *dev, size_t size,
31 dma_addr_t *dma_handle, gfp_t flag)
32 33Consistent memory is memory for which a write by either the device or
34the processor can immediately be read by the processor or device
35without having to worry about caching effects. (You may however need
36to make sure to flush the processor's write buffers before telling
37devices to read that memory.)
38 39This routine allocates a region of <size> bytes of consistent memory.
40 41It returns a pointer to the allocated region (in the processor's virtual
42address space) or NULL if the allocation failed.
43 44It also returns a <dma_handle> which may be cast to an unsigned integer the
45same width as the bus and given to the device as the DMA address base of
46the region.
47 48Note: consistent memory can be expensive on some platforms, and the
49minimum allocation length may be as big as a page, so you should
50consolidate your requests for consistent memory as much as possible.
51The simplest way to do that is to use the dma_pool calls (see below).
52 53The flag parameter (dma_alloc_coherent() only) allows the caller to
54specify the GFP_ flags (see kmalloc()) for the allocation (the
55implementation may choose to ignore flags that affect the location of
56the returned memory, like GFP_DMA).
57 58void *
59dma_zalloc_coherent(struct device *dev, size_t size,
60 dma_addr_t *dma_handle, gfp_t flag)
61 62Wraps dma_alloc_coherent() and also zeroes the returned memory if the
63allocation attempt succeeded.
64 65void
66dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
67 dma_addr_t dma_handle)
68 69Free a region of consistent memory you previously allocated. dev,
70size and dma_handle must all be the same as those passed into
71dma_alloc_coherent(). cpu_addr must be the virtual address returned by
72the dma_alloc_coherent().
73 74Note that unlike their sibling allocation calls, these routines
75may only be called with IRQs enabled.
76 77 78Part Ib - Using small DMA-coherent buffers
79------------------------------------------
80 81To get this part of the dma_ API, you must #include <linux/dmapool.h>
82 83Many drivers need lots of small DMA-coherent memory regions for DMA
84descriptors or I/O buffers. Rather than allocating in units of a page
85or more using dma_alloc_coherent(), you can use DMA pools. These work
86much like a struct kmem_cache, except that they use the DMA-coherent allocator,
87not __get_free_pages(). Also, they understand common hardware constraints
88for alignment, like queue heads needing to be aligned on N-byte boundaries.
89 90 91 struct dma_pool *
92 dma_pool_create(const char *name, struct device *dev,
93 size_t size, size_t align, size_t alloc);
94 95dma_pool_create() initializes a pool of DMA-coherent buffers
96for use with a given device. It must be called in a context which
97can sleep.
98 99The "name" is for diagnostics (like a struct kmem_cache name); dev and size
100are like what you'd pass to dma_alloc_coherent(). The device's hardware
101alignment requirement for this type of data is "align" (which is expressed
102in bytes, and must be a power of two). If your device has no boundary
103crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated
104from this pool must not cross 4KByte boundaries.
105 106 107 void *dma_pool_zalloc(struct dma_pool *pool, gfp_t mem_flags,
108 dma_addr_t *handle)
109 110Wraps dma_pool_alloc() and also zeroes the returned memory if the
111allocation attempt succeeded.
112 113 114 void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
115 dma_addr_t *dma_handle);
116 117This allocates memory from the pool; the returned memory will meet the
118size and alignment requirements specified at creation time. Pass
119GFP_ATOMIC to prevent blocking, or if it's permitted (not
120in_interrupt, not holding SMP locks), pass GFP_KERNEL to allow
121blocking. Like dma_alloc_coherent(), this returns two values: an
122address usable by the CPU, and the DMA address usable by the pool's
123device.
124 125 126 void dma_pool_free(struct dma_pool *pool, void *vaddr,
127 dma_addr_t addr);
128 129This puts memory back into the pool. The pool is what was passed to
130dma_pool_alloc(); the CPU (vaddr) and DMA addresses are what
131were returned when that routine allocated the memory being freed.
132 133 134 void dma_pool_destroy(struct dma_pool *pool);
135 136dma_pool_destroy() frees the resources of the pool. It must be
137called in a context which can sleep. Make sure you've freed all allocated
138memory back to the pool before you destroy it.
139 140 141Part Ic - DMA addressing limitations
142------------------------------------
143 144int
145dma_set_mask_and_coherent(struct device *dev, u64 mask)
146 147Checks to see if the mask is possible and updates the device
148streaming and coherent DMA mask parameters if it is.
149 150Returns: 0 if successful and a negative error if not.
151 152int
153dma_set_mask(struct device *dev, u64 mask)
154 155Checks to see if the mask is possible and updates the device
156parameters if it is.
157 158Returns: 0 if successful and a negative error if not.
159 160int
161dma_set_coherent_mask(struct device *dev, u64 mask)
162 163Checks to see if the mask is possible and updates the device
164parameters if it is.
165 166Returns: 0 if successful and a negative error if not.
167 168u64
169dma_get_required_mask(struct device *dev)
170 171This API returns the mask that the platform requires to
172operate efficiently. Usually this means the returned mask
173is the minimum required to cover all of memory. Examining the
174required mask gives drivers with variable descriptor sizes the
175opportunity to use smaller descriptors as necessary.
176 177Requesting the required mask does not alter the current mask. If you
178wish to take advantage of it, you should issue a dma_set_mask()
179call to set the mask to the value returned.
180 181 182Part Id - Streaming DMA mappings
183--------------------------------
184 185dma_addr_t
186dma_map_single(struct device *dev, void *cpu_addr, size_t size,
187 enum dma_data_direction direction)
188 189Maps a piece of processor virtual memory so it can be accessed by the
190device and returns the DMA address of the memory.
191 192The direction for both APIs may be converted freely by casting.
193However the dma_ API uses a strongly typed enumerator for its
194direction:
195 196DMA_NONE no direction (used for debugging)
197DMA_TO_DEVICE data is going from the memory to the device
198DMA_FROM_DEVICE data is coming from the device to the memory
199DMA_BIDIRECTIONAL direction isn't known
200 201Notes: Not all memory regions in a machine can be mapped by this API.
202Further, contiguous kernel virtual space may not be contiguous as
203physical memory. Since this API does not provide any scatter/gather
204capability, it will fail if the user tries to map a non-physically
205contiguous piece of memory. For this reason, memory to be mapped by
206this API should be obtained from sources which guarantee it to be
207physically contiguous (like kmalloc).
208 209Further, the DMA address of the memory must be within the
210dma_mask of the device (the dma_mask is a bit mask of the
211addressable region for the device, i.e., if the DMA address of
212the memory ANDed with the dma_mask is still equal to the DMA
213address, then the device can perform DMA to the memory). To
214ensure that the memory allocated by kmalloc is within the dma_mask,
215the driver may specify various platform-dependent flags to restrict
216the DMA address range of the allocation (e.g., on x86, GFP_DMA
217guarantees to be within the first 16MB of available DMA addresses,
218as required by ISA devices).
219 220Note also that the above constraints on physical contiguity and
221dma_mask may not apply if the platform has an IOMMU (a device which
222maps an I/O DMA address to a physical memory address). However, to be
223portable, device driver writers may *not* assume that such an IOMMU
224exists.
225 226Warnings: Memory coherency operates at a granularity called the cache
227line width. In order for memory mapped by this API to operate
228correctly, the mapped region must begin exactly on a cache line
229boundary and end exactly on one (to prevent two separately mapped
230regions from sharing a single cache line). Since the cache line size
231may not be known at compile time, the API will not enforce this
232requirement. Therefore, it is recommended that driver writers who
233don't take special care to determine the cache line size at run time
234only map virtual regions that begin and end on page boundaries (which
235are guaranteed also to be cache line boundaries).
236 237DMA_TO_DEVICE synchronisation must be done after the last modification
238of the memory region by the software and before it is handed off to
239the device. Once this primitive is used, memory covered by this
240primitive should be treated as read-only by the device. If the device
241may write to it at any point, it should be DMA_BIDIRECTIONAL (see
242below).
243 244DMA_FROM_DEVICE synchronisation must be done before the driver
245accesses data that may be changed by the device. This memory should
246be treated as read-only by the driver. If the driver needs to write
247to it at any point, it should be DMA_BIDIRECTIONAL (see below).
248 249DMA_BIDIRECTIONAL requires special handling: it means that the driver
250isn't sure if the memory was modified before being handed off to the
251device and also isn't sure if the device will also modify it. Thus,
252you must always sync bidirectional memory twice: once before the
253memory is handed off to the device (to make sure all memory changes
254are flushed from the processor) and once before the data may be
255accessed after being used by the device (to make sure any processor
256cache lines are updated with data that the device may have changed).
257 258void
259dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
260 enum dma_data_direction direction)
261 262Unmaps the region previously mapped. All the parameters passed in
263must be identical to those passed in (and returned) by the mapping
264API.
265 266dma_addr_t
267dma_map_page(struct device *dev, struct page *page,
268 unsigned long offset, size_t size,
269 enum dma_data_direction direction)
270void
271dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
272 enum dma_data_direction direction)
273 274API for mapping and unmapping for pages. All the notes and warnings
275for the other mapping APIs apply here. Also, although the <offset>
276and <size> parameters are provided to do partial page mapping, it is
277recommended that you never use these unless you really know what the
278cache width is.
279 280dma_addr_t
281dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size,
282 enum dma_data_direction dir, unsigned long attrs)
283 284void
285dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size,
286 enum dma_data_direction dir, unsigned long attrs)
287 288API for mapping and unmapping for MMIO resources. All the notes and
289warnings for the other mapping APIs apply here. The API should only be
290used to map device MMIO resources, mapping of RAM is not permitted.
291 292int
293dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
294 295In some circumstances dma_map_single(), dma_map_page() and dma_map_resource()
296will fail to create a mapping. A driver can check for these errors by testing
297the returned DMA address with dma_mapping_error(). A non-zero return value
298means the mapping could not be created and the driver should take appropriate
299action (e.g. reduce current DMA mapping usage or delay and try again later).
300 301 int
302 dma_map_sg(struct device *dev, struct scatterlist *sg,
303 int nents, enum dma_data_direction direction)
304 305Returns: the number of DMA address segments mapped (this may be shorter
306than <nents> passed in if some elements of the scatter/gather list are
307physically or virtually adjacent and an IOMMU maps them with a single
308entry).
309 310Please note that the sg cannot be mapped again if it has been mapped once.
311The mapping process is allowed to destroy information in the sg.
312 313As with the other mapping interfaces, dma_map_sg() can fail. When it
314does, 0 is returned and a driver must take appropriate action. It is
315critical that the driver do something, in the case of a block driver
316aborting the request or even oopsing is better than doing nothing and
317corrupting the filesystem.
318 319With scatterlists, you use the resulting mapping like this:
320 321 int i, count = dma_map_sg(dev, sglist, nents, direction);
322 struct scatterlist *sg;
323 324 for_each_sg(sglist, sg, count, i) {
325 hw_address[i] = sg_dma_address(sg);
326 hw_len[i] = sg_dma_len(sg);
327 }
328 329where nents is the number of entries in the sglist.
330 331The implementation is free to merge several consecutive sglist entries
332into one (e.g. with an IOMMU, or if several pages just happen to be
333physically contiguous) and returns the actual number of sg entries it
334mapped them to. On failure 0, is returned.
335 336Then you should loop count times (note: this can be less than nents times)
337and use sg_dma_address() and sg_dma_len() macros where you previously
338accessed sg->address and sg->length as shown above.
339 340 void
341 dma_unmap_sg(struct device *dev, struct scatterlist *sg,
342 int nents, enum dma_data_direction direction)
343 344Unmap the previously mapped scatter/gather list. All the parameters
345must be the same as those and passed in to the scatter/gather mapping
346API.
347 348Note: <nents> must be the number you passed in, *not* the number of
349DMA address entries returned.
350 351void
352dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
353 enum dma_data_direction direction)
354void
355dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size,
356 enum dma_data_direction direction)
357void
358dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nents,
359 enum dma_data_direction direction)
360void
361dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nents,
362 enum dma_data_direction direction)
363 364Synchronise a single contiguous or scatter/gather mapping for the CPU
365and device. With the sync_sg API, all the parameters must be the same
366as those passed into the single mapping API. With the sync_single API,
367you can use dma_handle and size parameters that aren't identical to
368those passed into the single mapping API to do a partial sync.
369 370Notes: You must do this:
371 372- Before reading values that have been written by DMA from the device
373 (use the DMA_FROM_DEVICE direction)
374- After writing values that will be written to the device using DMA
375 (use the DMA_TO_DEVICE) direction
376- before *and* after handing memory to the device if the memory is
377 DMA_BIDIRECTIONAL
378 379See also dma_map_single().
380 381dma_addr_t
382dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
383 enum dma_data_direction dir,
384 unsigned long attrs)
385 386void
387dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
388 size_t size, enum dma_data_direction dir,
389 unsigned long attrs)
390 391int
392dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
393 int nents, enum dma_data_direction dir,
394 unsigned long attrs)
395 396void
397dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
398 int nents, enum dma_data_direction dir,
399 unsigned long attrs)
400 401The four functions above are just like the counterpart functions
402without the _attrs suffixes, except that they pass an optional
403dma_attrs.
404 405The interpretation of DMA attributes is architecture-specific, and
406each attribute should be documented in Documentation/DMA-attributes.txt.
407 408If dma_attrs are 0, the semantics of each of these functions
409is identical to those of the corresponding function
410without the _attrs suffix. As a result dma_map_single_attrs()
411can generally replace dma_map_single(), etc.
412 413As an example of the use of the *_attrs functions, here's how
414you could pass an attribute DMA_ATTR_FOO when mapping memory
415for DMA:
416 417#include <linux/dma-mapping.h>
418/* DMA_ATTR_FOO should be defined in linux/dma-mapping.h and
419 * documented in Documentation/DMA-attributes.txt */
420...
421 422 unsigned long attr;
423 attr |= DMA_ATTR_FOO;
424 ....
425 n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, attr);
426 ....
427 428Architectures that care about DMA_ATTR_FOO would check for its
429presence in their implementations of the mapping and unmapping
430routines, e.g.:
431 432void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
433 size_t size, enum dma_data_direction dir,
434 unsigned long attrs)
435{
436 ....
437 if (attrs & DMA_ATTR_FOO)
438 /* twizzle the frobnozzle */
439 ....
440 441 442Part II - Advanced dma_ usage
443-----------------------------
444 445Warning: These pieces of the DMA API should not be used in the
446majority of cases, since they cater for unlikely corner cases that
447don't belong in usual drivers.
448 449If you don't understand how cache line coherency works between a
450processor and an I/O device, you should not be using this part of the
451API at all.
452 453void *
454dma_alloc_noncoherent(struct device *dev, size_t size,
455 dma_addr_t *dma_handle, gfp_t flag)
456 457Identical to dma_alloc_coherent() except that the platform will
458choose to return either consistent or non-consistent memory as it sees
459fit. By using this API, you are guaranteeing to the platform that you
460have all the correct and necessary sync points for this memory in the
461driver should it choose to return non-consistent memory.
462 463Note: where the platform can return consistent memory, it will
464guarantee that the sync points become nops.
465 466Warning: Handling non-consistent memory is a real pain. You should
467only use this API if you positively know your driver will be
468required to work on one of the rare (usually non-PCI) architectures
469that simply cannot make consistent memory.
470 471void
472dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
473 dma_addr_t dma_handle)
474 475Free memory allocated by the nonconsistent API. All parameters must
476be identical to those passed in (and returned by
477dma_alloc_noncoherent()).
478 479int
480dma_get_cache_alignment(void)
481 482Returns the processor cache alignment. This is the absolute minimum
483alignment *and* width that you must observe when either mapping
484memory or doing partial flushes.
485 486Notes: This API may return a number *larger* than the actual cache
487line, but it will guarantee that one or more cache lines fit exactly
488into the width returned by this call. It will also always be a power
489of two for easy alignment.
490 491void
492dma_cache_sync(struct device *dev, void *vaddr, size_t size,
493 enum dma_data_direction direction)
494 495Do a partial sync of memory that was allocated by
496dma_alloc_noncoherent(), starting at virtual address vaddr and
497continuing on for size. Again, you *must* observe the cache line
498boundaries when doing this.
499 500int
501dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
502 dma_addr_t device_addr, size_t size, int
503 flags)
504 505Declare region of memory to be handed out by dma_alloc_coherent() when
506it's asked for coherent memory for this device.
507 508phys_addr is the CPU physical address to which the memory is currently
509assigned (this will be ioremapped so the CPU can access the region).
510 511device_addr is the DMA address the device needs to be programmed
512with to actually address this memory (this will be handed out as the
513dma_addr_t in dma_alloc_coherent()).
514 515size is the size of the area (must be multiples of PAGE_SIZE).
516 517flags can be ORed together and are:
518 519DMA_MEMORY_MAP - request that the memory returned from
520dma_alloc_coherent() be directly writable.
521 522DMA_MEMORY_IO - request that the memory returned from
523dma_alloc_coherent() be addressable using read()/write()/memcpy_toio() etc.
524 525One or both of these flags must be present.
526 527DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by
528dma_alloc_coherent of any child devices of this one (for memory residing
529on a bridge).
530 531DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions.
532Do not allow dma_alloc_coherent() to fall back to system memory when
533it's out of memory in the declared region.
534 535The return value will be either DMA_MEMORY_MAP or DMA_MEMORY_IO and
536must correspond to a passed in flag (i.e. no returning DMA_MEMORY_IO
537if only DMA_MEMORY_MAP were passed in) for success or zero for
538failure.
539 540Note, for DMA_MEMORY_IO returns, all subsequent memory returned by
541dma_alloc_coherent() may no longer be accessed directly, but instead
542must be accessed using the correct bus functions. If your driver
543isn't prepared to handle this contingency, it should not specify
544DMA_MEMORY_IO in the input flags.
545 546As a simplification for the platforms, only *one* such region of
547memory may be declared per device.
548 549For reasons of efficiency, most platforms choose to track the declared
550region only at the granularity of a page. For smaller allocations,
551you should use the dma_pool() API.
552 553void
554dma_release_declared_memory(struct device *dev)
555 556Remove the memory region previously declared from the system. This
557API performs *no* in-use checking for this region and will return
558unconditionally having removed all the required structures. It is the
559driver's job to ensure that no parts of this memory region are
560currently in use.
561 562void *
563dma_mark_declared_memory_occupied(struct device *dev,
564 dma_addr_t device_addr, size_t size)
565 566This is used to occupy specific regions of the declared space
567(dma_alloc_coherent() will hand out the first free region it finds).
568 569device_addr is the *device* address of the region requested.
570 571size is the size (and should be a page-sized multiple).
572 573The return value will be either a pointer to the processor virtual
574address of the memory, or an error (via PTR_ERR()) if any part of the
575region is occupied.
576 577Part III - Debug drivers use of the DMA-API
578-------------------------------------------
579 580The DMA-API as described above has some constraints. DMA addresses must be
581released with the corresponding function with the same size for example. With
582the advent of hardware IOMMUs it becomes more and more important that drivers
583do not violate those constraints. In the worst case such a violation can
584result in data corruption up to destroyed filesystems.
585 586To debug drivers and find bugs in the usage of the DMA-API checking code can
587be compiled into the kernel which will tell the developer about those
588violations. If your architecture supports it you can select the "Enable
589debugging of DMA-API usage" option in your kernel configuration. Enabling this
590option has a performance impact. Do not enable it in production kernels.
591 592If you boot the resulting kernel will contain code which does some bookkeeping
593about what DMA memory was allocated for which device. If this code detects an
594error it prints a warning message with some details into your kernel log. An
595example warning message may look like this:
596 597------------[ cut here ]------------
598WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
599 check_unmap+0x203/0x490()
600Hardware name:
601forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
602 function [device address=0x00000000640444be] [size=66 bytes] [mapped as
603single] [unmapped as page]
604Modules linked in: nfsd exportfs bridge stp llc r8169
605Pid: 0, comm: swapper Tainted: G W 2.6.28-dmatest-09289-g8bb99c0 #1
606Call Trace:
607 <IRQ> [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
608 [<ffffffff80647b70>] _spin_unlock+0x10/0x30
609 [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
610 [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
611 [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
612 [<ffffffff80252f96>] queue_work+0x56/0x60
613 [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
614 [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
615 [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
616 [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
617 [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
618 [<ffffffff803c7ea3>] check_unmap+0x203/0x490
619 [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
620 [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
621 [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
622 [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
623 [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
624 [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
625 [<ffffffff8020c093>] ret_from_intr+0x0/0xa
626 <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
627 628The driver developer can find the driver and the device including a stacktrace
629of the DMA-API call which caused this warning.
630 631Per default only the first error will result in a warning message. All other
632errors will only silently counted. This limitation exist to prevent the code
633from flooding your kernel log. To support debugging a device driver this can
634be disabled via debugfs. See the debugfs interface documentation below for
635details.
636 637The debugfs directory for the DMA-API debugging code is called dma-api/. In
638this directory the following files can currently be found:
639 640 dma-api/all_errors This file contains a numeric value. If this
641 value is not equal to zero the debugging code
642 will print a warning for every error it finds
643 into the kernel log. Be careful with this
644 option, as it can easily flood your logs.
645 646 dma-api/disabled This read-only file contains the character 'Y'
647 if the debugging code is disabled. This can
648 happen when it runs out of memory or if it was
649 disabled at boot time
650 651 dma-api/error_count This file is read-only and shows the total
652 numbers of errors found.
653 654 dma-api/num_errors The number in this file shows how many
655 warnings will be printed to the kernel log
656 before it stops. This number is initialized to
657 one at system boot and be set by writing into
658 this file
659 660 dma-api/min_free_entries
661 This read-only file can be read to get the
662 minimum number of free dma_debug_entries the
663 allocator has ever seen. If this value goes
664 down to zero the code will disable itself
665 because it is not longer reliable.
666 667 dma-api/num_free_entries
668 The current number of free dma_debug_entries
669 in the allocator.
670 671 dma-api/driver-filter
672 You can write a name of a driver into this file
673 to limit the debug output to requests from that
674 particular driver. Write an empty string to
675 that file to disable the filter and see
676 all errors again.
677 678If you have this code compiled into your kernel it will be enabled by default.
679If you want to boot without the bookkeeping anyway you can provide
680'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
681Notice that you can not enable it again at runtime. You have to reboot to do
682so.
683 684If you want to see debug messages only for a special device driver you can
685specify the dma_debug_driver=<drivername> parameter. This will enable the
686driver filter at boot time. The debug code will only print errors for that
687driver afterwards. This filter can be disabled or changed later using debugfs.
688 689When the code disables itself at runtime this is most likely because it ran
690out of dma_debug_entries. These entries are preallocated at boot. The number
691of preallocated entries is defined per architecture. If it is too low for you
692boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
693architectural default.
694 695void debug_dmap_mapping_error(struct device *dev, dma_addr_t dma_addr);
696 697dma-debug interface debug_dma_mapping_error() to debug drivers that fail
698to check DMA mapping errors on addresses returned by dma_map_single() and
699dma_map_page() interfaces. This interface clears a flag set by
700debug_dma_map_page() to indicate that dma_mapping_error() has been called by
701the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
702this flag is still set, prints warning message that includes call trace that
703leads up to the unmap. This interface can be called from dma_mapping_error()
704routines to enable DMA mapping error check debugging.
705 706